The acid alkylation of isoparaffins with olefins, such illustratively as propylene, butylenes, amylenes and the like, by charging to a standard commercial alkylation production unit selected olefins, isobutane, in great excess of the olefins, and strong sulfuric acid, wherein the charge of H.sub.2 SO.sub.4 is present in a weight percent concentration of 98.0 to 99.5, is well-known and widely used commercially. The reaction proceeds at about 30.degree. to 50.degree. F. and the reaction mixture is withdrawn from the reactor and separated into an acid phase and a hydrocarbon phase. Most of the acid phase is recycled to the reactor but the strong sulfuric acid used as a catalyst in the alkylation of the isobutane with aliphatic olefins gradually becomes contaminated with water, polymeric hydrocarbons and sulfate esters. As a consequence, the titratable acidity of the sulfuric acid catalyst drops from its original value so that to maintain the acid in the reaction system at the elevated concentration required for efficient catalysis, it becomes necessary to withdraw catalyst acid from the alkylation system at a rate sufficient to maintain the titratable acidity of the remaining acid in the system at a concentration by weight of about 85 to 92 wt. % and usually about 90 wt. %. This weight of withdrawal usually amounts to about 0.3 pound to 1.0 pound per gallon of alkylated produced. This withdrawn acid, characterized further in the description appearing hereinafter, is referred to as "used" or "spent alkylation acid" or briefly as "spent acid."
The composition of withdrawn spent alkylation acid does not vary appreciably with the olefin used, but the amount of spent acid recovered varies appreciably. In general the hydrocarbon content increases with decreasing titratable acidity.
While the amount of spent acid withdrawn is small on a unit weight basis, the quantities producted by commercial refinery facilities are large, and if not recovered lead to substantial emissions of noxious effluent to the surrounding atmosphere. One method of reducing these emissions by recovery of the used or spent acid is described, illustratively, in U.S. Pat. No. 3,773,917.
Also present, however, in petroleum refining complexes are fluid catalytic cracking regenerator systems yielding gases containing significant quantities, in the volumes produced, of carbon monoxide. Substantial amounts of carbon monoxide are also produced inter alia by separating carbon monoxide from synthesis gas, using a copper liquor absorbent, and from smelting operations.
At the same time and in operative proximity, for the purpose of the present invention, to refinery facilities including cracking, synthesis and alkylation units such as the foregoing, and smelting operations or the like are often disposed utility plant facilities engaged in the production of energy using fossil fuels, particularly high sulfur petroleum residual fuels or coal, the effluent from which, as normally evidenced by the stack gases emanating from these facilities, contain noxious amounts and concentrations of sulfur dioxide and nitrogen oxides.
Efforts made to remove sulfur dioxide have involved heretofore, in one manifestation, the employment of basic aqueous solutions and slurries which tend to leave significant concentrations of SO.sub.2 behind which then appear in the stack or flue gases, particularly where high sulfur residual petroleum fuels or coal are employed. However, unless complete removal of SO.sub.2 is effected, groundlevel concentrations of this pollutant will occur which may be only slightly better than those resulting from untreated flue gases.
The Tyco process has provided another means for removal of SO.sub.2, and nitrogen oxides (NxOy) from stack gases, utilizing a modification of the chamber process, now obsolescent, wherein nitrosyl sulfuric acid (HNSO.sub.5) is derived by the following reaction sequence: EQU SO.sub.2 + H.sub.2 O + NO.sub.2 .fwdarw.H.sub.2 SO.sub.4 + NO (1) EQU 2no + o.sub.2 .fwdarw.2no.sub.2 ( 2) EQU no + no.sub.2 + 2h.sub.2 so.sub.4 .fwdarw.2hnso.sub.5 + h.sub.2 o (3)
in these reactions NO.sub.2 acts as a homogeneous catalyst; and whereas steps (1) and (2) occur simultaneously in the conventional chamber reaction they are undertaken separately in the Tyco process. It was found advantageous to have these reactions performed separately since reaction (1) occurs much more rapidly than reaction (2). Thus, in the Tyco process this reaction (2) is carried out separately after stripping an equimolar mixture of nitric oxide and nitrogen dioxide (or N.sub.2 O.sub.3) from the nitrosyl sulfuric acid and cooling it. The excess NO.sub.2 is converted to nitric acid in the typical Tyco process.
In the original embodiment of the Tyco process, NO.sub.2 is introduced into flue gas to provide the reactant mixture of reaction (1) at 300.degree. F. with consequent and complete oxidation of SO.sub.2 in 5 seconds. Excess nitrogen oxides provide an oxidation level corresponding to N.sub.2 O.sub.3.
In this original or baseline process the reaction gas is countercurrently contacted with H.sub.2 SO.sub.4 of 80 wt. % concentration at 80.degree. F. Gas from the absorber goes to the stack. The scrubber exit liquid is nitrosyl sulfuric acid dissolved in 76 wt. % H.sub.2 SO.sub.4 at about 275.degree. F. It is reconstituted to 80 wt. % sulfuric acid at 395.degree. F. by hot combustion gas. Air oxidation converts the nitrogen oxides to NO.sub.2 in a re-oxidation chamber, and part is removed as nitric acid. The remainder is introduced into flue gas. The latter is newly treated in conjunction with this recycled remainder.
One modification of this initial embodiment involves the use of cooling to separate water upstream of the absorber, but this too was deemed unattractive commercially.
A further modification is similar to that of the original process up to the absorption step. Absorption is however, undertaken at an elevated temperature, i.e. about 250.degree. F. and N.sub.2 O.sub.3 is, as a result, recovered without condensation of water also present normally in the combustion or stack gases being treated. A solution of nitrosyl sulfuric acid in 80 wt. % acid concentration is filtered in this modification and passed to a catalytic reactor packed with charcoal, and is then passed countercurrently to air that strips and oxidizes the nitrogen oxides. Part of the NO.sub.2 is absorbed as HNO.sub.3 as in the earlier modifications, and the remainder also recycled as previously described.
The Tyco process thus involves basically a reliance on the recycling of nitrogen oxides to react with the effluent sulfur dioxide and water of reaction (1).
Indeed, conventionally as seen, the process requires the incorporation of nitric acid into reaction (1), a factor which tends to detract from the economic viability of the Tyco process.
Additionally, the Tyco process is not adapted to treat both flue gases and spent alkylation acid nor is it capable of accomplishing its espoused purpose in a single pass through, as indicated. Significantly, too, this process is capable normally of generating only dilute levels of sulfuric acid, rather than concentrated acid and immediately useful, more readily shipped products, such as sulfur.
Various other experimental processes have also been developed for recovery of one or more of the other pollutants referred to herein above, that is, carbon monoxide, nitrogen oxides, and the like.
If, accordingly, the pollutants incorporated in flue gases and refinery effluent emission from power plants utilizing fossil fuels; from petroleum refinery operations and additionally, metallic ore smelters as well as other petroleum product sources, could be mutually entrained and subjected in large volume to an efficient process of absorption, separation and reaction in which the several pollutants were utilized to aid in recovery of significant industrial chemicals and the formation of innocuous effluent which could be let free in the surrounding atmosphere without significant concern for its pollutant effect, and without the required recycling of impurities, a valuable and economic method would be effected which would constitute a significant advance in the state of the art.